This invention generally relates to improvements in continuous flow density meters.
The standard method for precisely determining the density of fluids involves the use of a closed pressure vessel, commonly known as a pycnometer, whose volume is precisely determined, and of an accurate weighing device. The weighing device is calibrated using standard weights traceable to the National Bureau of Standards (NBS). The pycnometer consists of a valved bulb of known volume that is filled with the sample, valve closed and weighed on the weighing device. The volume of the pycnometer bulb is corrected for the temperature and pressure of the sample; that is, as the temperature and pressure increase, the walls of the bulb expand, causing an increase in volume that must be considered for accurate density calculations. The volume calibration of the pycnometer is generally described in a bulletin published by the American Petroleum Institute; API MPMS Chapter 11.2.3- "Water Calibration of Volumetric Provers." This method is related to the volume calibration method described below for the continuous density meter described herein.
The direct weigh method is accepted as a batch method for determining density, but until the present invention has not been practical for continuous monitoring of density. A major problem has been that when the pycnometer is connected to a source for continuous flow, the flow connections interfere with the accuracy of the weighing device. The flow connections must be able to withstand high pressures, extremes of temperatures, and be of a large enough diameter to accomodate a high flow in order to continuously change the sample in the bulb. The bulb must be of large enough size that small change in density may be resolved. Flexible connections either do not tolerate these hostile conditions or they produce twisting forces that interfere with accurate weighing. Rigid connections do not allow free movement of the bulb or develop resonant vibrations that cause oscillating feedback in the weighing device.
SUMMARY OF THE INVENTION
An object of this invention is to provide means for producing signal that is proportional to the weight of a continuously flowing sample of fluid.
A further object of this invention is to provide means by which a continuously flowing sample, the weight of which is to be accurately determined, may be passed through the instrument at the same pressure as the source.
Another object of this invention is to provide means by which a continuously flowing sample, the weight of which is to be accurately determined, may be passed through the instrument at the same temperature as the source.
Another object of this invention is to provide means for producing a signal that is proportional to the volume of a continuously flowing sample of fluid within a range of temperature and pressure combinations.
Another object of this invention is to provide means for processing the first signal that is proportional to the instantaneous weight of a sample and the second signal that is proportional to the instananeous volume of said sample, in order to produce a third signal that is proportional to the instantaneous density of said sample.
Another object of the invention is to provide means for determining the density of a continuously flowing fluid that is calibrated directly with weight standards traceable to the National Bureau of Standards.
Other objects of the invention will appear as the description thereof proceeds.
In the present invention the connection problem is solved by using the connections as the weighing device; that is, the main flexures of the weighing device are hollow and serve the dual purpose of flexures and flow connections. In this regard, U.S. Pat. No. 2,231,175 describes a mechanical device that uses "spring tubes" to partially support the bulb. In the present invention the spring tube concept is refined and improved; and it is combined with devices for electronically determining displacement of the flexures, temperatures and pressure and with microprocessor based signal processing. The end result is a continuous density meter that is highly accurate and reliable, while remaining based on a simple, straight-forward principle of operation.
The invention provides a continuous density meter system which is mechanically simple in construction, relatively inexpensive in cost of manufacture and assembly, and generally superior to continuous density meters now known to applicant in that it is based on the straight-forward concept of determining the weight and the volume of the sample, and of calculating density by simple arithmetic division. It is the only continuous density meter that utilizes the direct weight technique, to my knowledge.
A further object comprises the novel and useful provision, formation, construction, association and relative arrangements of parts, members, and features, all as depicted in a certain embodment in the accompanying drawings, described generally, and more particularly pointed out in the claims.